Prior techniques and arrangements for converting synchronous digital transmission signals to asynchronous digital signals are known. In recent digital transmission systems, it has become important to smooth large gaps in a supplied data signal resulting from overhead bit removal and stuff bits. This is particularly important, for example, in converting a SONET STS-1 synchronous digital signal to a DS3 asynchronous digital signal. As is known, pointer adjustments are used in the STS-1 signal format to reconcile small phase and frequency differences between a clock signal derived from an incoming STS-1 signal and a local clock signal. These pointer adjustments are made on a byte-wise basis and can be either positive or negative. During normal system operation, the pointer adjustments occur relatively infrequently. This causes a low frequency, relatively large peak-to-peak jitter component in the desynchronized signal. When the system operation is degraded, pointer adjustments may occur more often, and randomly occurring pointer adjustments may be superimposed on the normal periodic sequence of pointer adjustments. This superposition of randomly occurring pointer adjustments can cause the cancellation of one or more pointer adjustments of and/or the addition of one or more pointer adjustments to the periodically occurring pointer adjustments. Such occurrences are undesirable from a jitter performance point of view.
A bit leaking technique in conjunction with a phase locked loop and a desynchronizing elastic store has been proposed in an attempt at smoothing gaps in a data signal caused by the pointer adjustments in a SONET signal format. A bit leak is defined as one bit of phase error being supplied to a phase locked loop. One of these techniques employs a bit-by-bit leaking adjustments so that a phase locked loop having a "wider" bandwidth may be employed in the desynchronizer. This bit-by-bit technique, when constrained by a payload output jitter specification to be satisfied, however, does not adequately compensate for the full range of pointer adjustment rates which may occur.
More recently, as disclosed in U.S. Pat. No. 5,052,025 issued on Sep. 24, 1991, an open loop estimation averaging arrangement has been employed to derive a bit leaking interval. One problem with the open loop estimation averaging technique is that the average bit leaking interval is only updated on detection of the next received pointer adjustment. This prior arrangement also employs a centering scheme for an associated desynchronizing elastic store. Such a scheme can be a potential source of significant jitter in the data signal output of the elastic store. Although this prior known technique operates satisfactorily in normal pointer adjustment operation, its performance is not entirely satisfactory in a degraded mode of operation. Such a degraded mode of operation occurs in the presence of random pointer adjustments and the superposition of the random pointer adjustments on the periodically received pointer adjustments.